I. Field of the Invention
The present invention relates to a positive-type photosensitive resin composition suited for forming light-blocking separators and black matrices of organic electroluminescent devices and liquid crystal display elements, which are used in flat panel displays.
II. Description of the Related Art
As flat panel displays, liquid crystal displays (LCDs) which are non-luminescent have been well spread. On the other hand, organic electroluminescent devices used in self-luminescent displays are now widely studied because high brightness can be obtained, and full-color displays can be attained.
Organic electroluminescent devices are operated by applying electric voltage or electric current between a first electrode and a second electrode. When operating an electroluminescent device, since electric field tends to be concentrated to the edge portions of the electrodes, which have small radii of curvature, undesirable phenomena such as dielectric breakdown and generation of leak current are likely to occur at the edge portions of the electrodes.
To reduce these phenomena, it is known to cover the edge portion of the first electrode with an insulation layer (also called separator) (U.S. Pat. No. 6,222,315).
By this technique, concentration of electric field to the edge portion of the electrode can be reduced. Further, the insulation layer may be formed such that the thickness of the insulation layer at the exposed boundary region of the first electrode is larger and larger as the distance from the boundary is larger and larger (see FIG. 1). In other words, the insulation layer can be formed so as to have a “forward-tapered” cross-section. By virtue of this configuration, concentration of the electric field at the edge portion of the electrode is further reduced. Recently, it is tried to make the insulation films and/or element-separating structures have light-blocking properties by adding carbon black to non-photosensitive polyimide resins (Japanese Laid-open Patent Application (Kokai) No. 11-273870). Known materials for constituting the black matrices include negative-type photosensitive materials (U.S. Pat. No. 6,171,733) comprising alkali-soluble resins obtained by polycondensing phenols and aldehydes, acid-crosslinking methylolated melamine compounds, photoacid generators, particular dispersing agents and black pigments; and non-photosensitive materials (U.S. Pat. Nos. 5,780,201 and 5,998,090) comprising polyimide precursors, vehicles therefor, solvents, light-absorbing dyes or mixtures thereof, carbon black pigment or non-carbon black metal oxide pigments and Newton dispersing agents.
In general, as the materials constituting the insulation layers in displays, polyimides, novolaks, acrylic resins and the like are used. These resins include non-photosensitive, negative-type photosensitive and positive-type photosensitive resins. To reduce the concentration of the electric field at the edge portions of the electrodes, positive-type photosensitive resins are preferred. Usually, the effective intensity of ray inside a coating film during exposure decreases as the distance from the surface of the coating film increases. Therefore, by using a positive-type photosensitive resin with which the exposed regions are dissolved, it is easier to form the forward-tapered shape of the cross-section of the exposed region.
A method for preparing a positive-type photosensitive resin composition for black matrices of LCDs is known, in which a quinone diazide compound and a black pigment are added to an alkali-soluble resin such as novolak resin or vinyl polymer (Japanese Laid-open Patent Application (Kokai) No. 6-230215). A method for preparing a light-blocking insulation film for organic electroluminescent devices is known, in which a quinone diazide compound and a coloring agent are added to an alkali-soluble resin such as novolak resin or a polymer of a radical-polymerizable monomer (Japanese Laid-open Patent Application (Kokai) No. 2002-116536).
However, the light-blocking positive-type photosensitive resin compositions prepared by adding coloring agents to the above-mentioned resin compositions have absorption maximum within the wavelength range of 350 nm to 450 nm, which is the wavelength range of mercury lamps usually used as the light sources for exposure, in order to attain sufficient light-blocking property. Therefore, they have a drawback in that the exposure sensitivity is decreased.
On the other hand, a method is known by which the light transmittance in the visible region (wavelength of 400 nm to 700 nm) may be decreased to not more than about 10%, in which a pigment is added to a photoresist for black matrices (Japanese Laid-open Patent Application (Kokai) No. 8-137098). In this method, as the means for coloring, a photoresist containing a cresol-novolak resin and a naphthoquinone diazide photosensitizer is used.
A method for giving light-blocking property to an insulation layer is known, in which a thermosensitive material and a developing agent are added to a positive-type photoresist based on diazoquinone-novolak resin (Japanese Laid-open Patent Application (Kokai) No. 10-170715). By this method, the photosensitive resin does not have light-blocking property before the exposure so that the exposure sensitivity is not decreased, since a positive-type photosensitive resin containing a thermodsensitive material which colors upon being heated and a developing agent is used and since the thermosensitive material and the developing agent are not reacted before the exposure and so the resin is not black before the exposure. However, since ordinary heatsensitive materials consisting of developing agents and color formers have coloring temperatures of 100° C. to 180° C., they have a problem in the heat-resistance in the final heat treatment (the temperature is usually not lower than 200° C.) after patterning of the coating film. That is, the thermosensitive materials which colored by heat are faded by the final heat-treatment, so that sufficient light-blocking property cannot be obtained. Further, the black color formed by using an ordinary thermosensitive material containing a developing agent and a color former has a problem in the light resistance after the final heat-treatment, that is, the black color is likely to fade upon irradiation with visible and UV lights for a long period of time, so that it is difficult to continuously use the black resin as a black matrix of a display apparatus.